The type three and four engines have different cooling setups, still aircooled but the fan is attached to the end of the crank, and it pushes air horizontally. The airflow will have to travel through several turns,
90 degree ones even, before it reaches the area it's supposed to cool. In order to reach the furthest corners of the target area, there has to be resistance to the flow. Air pressure will go the route of LEAST resistance, so if you have a gaping hole somewhere in the path before it reaches the intended destination, it will rather choose the easier route. That's where the type 3 cooling tin comes to play. It's there to block the "easier route".The very same theory is actually working in a type 1 engine (upright cooling, as opposed to horizontal) but there the airflow from the fan has less corners to negotiate. Type 1 heads and cylinders have the separate, smaller "blocking" plates attached to the iunderside of them, and they are enough to provide the necessary restriction in the otherwise easiest path. The restriction is needed to cooling air would be forced to reach teh furthest corners of teh cylinder head, that means a VERY tight 90 degree turn front and back as the air exits the fan shroud. If it was allowed to blow straight down, you would have asuper cool center section of the heads, and red hot exhaust port area on both sides. (Exaggerating a little here to make my point)
I could add here a piece on cooling flaps and why they are so critical even when they are in the open position.. wonder if I should... ah hell why not:
The cooling flaps, when closed, allow the engine to warm up properly, in the very carefully thought out, pre-determined sequence. This is critical because teh design of the engine is a sandwich of different metals that have VERY different thermal expansion rates. Enough of that, that's good enough reason to KEEP the flaps and thermostat operational. Now, when teh engine has reached it's normal operating temperature, and teh flaps are fully open.. they aren't just folded out of the way and sitting there doing nothing, they actually keep directing air precisely where it is needed most. The most heat is generated in the heads, where the combustion takes place. The flap assemblies on both sides have two flaps each. The flap closer to the center of the engine is less critical IMHO, it's role is pretty much to change or keep the direction of airflow vertical. This air goes solely to the cylinder fin area.
The outer flaps however, when they are fully open, actually serve a double purpose. When they open the air passage to the heads, they also CLOSE another air gap. The bottom edge of the outer flaps is lined up with the last cooling fin of the head (or close) and when the flap is pointing straight down (fully open) it damn near touches teh last, continuous cooling fin, preventing air from escaping from the head fin area. Now the air that comes from the shroud above, has only two directions to go to.. front and back of the engine, making a 90 degree turn. If this flap is missing, the air that is pushed towards the head fin area would once again take the path of least resistance, teh cooler and more open space of the cylinders, straight below. Even the air that would otherwise "naturally" be directed towards the head fins, will try to MAKE a turn back towards the center of the engine, and escape from between the cylinders. (Remember why you have the cylinder bottom clip-on tins pieces in there..) The airspace under the tin, where the HEAD resides, is considerably hotter to begin with and this mass (air has mass) of HOT air already there is making it hard for new, cool air to enter. It's like an invisible barrier (ask your weatherman for the theory)
In other words, leaving out any critical part from the cooling system is most likely to cause a lot of damage. The whole system was designed to work together as a whole, and does it VERY well when everythingt ios as it should be. No backyard mechanic can "improve" the cooling by yanking stuff out.
It's late, and I'm getting angry at general ignorance and for constantly having to explain this to people who refuse to believe and claim they or their redneck mechanic knows better than me (Or the trained engineers of VW in Wolfsburg who managed to make the same aircooled engine outlast anything out there both in Siberia and Sahara desert) and therefore I'll stop here. Do NOT challenge me on this, I will either not take the bait, or worse, I will have my wife stomp your ego to the ground and grind it down to dog food. And you know what dog food turns to at the end of it's life cycle. You don't want to be that.
Right. I think someone else could pick it up from here, I've barely touched the surface of our beloved aircooled VW engine and it's cooling system.
Jan